Characterizations of ZnO/MoO$_{\mathrm{3}}$ superlattices grown by Atomic Layer Deposition

ZnO/MoO$_{\mathrm{3}}$ superlattices (SLS) were prepared by ALD on Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ substrates at 450K. The growth rates are 0.17 per cycle for MoO$_{\mathrm{3}}$ and 1.66 for ZnO, according to XRR. The MoO$_{\mathrm{3}}$ films were found amorphous when deposited separately, while ZnO polycrystalline. However MoO$_{\mathrm{3}}$ became polycrystalline and ZnO textured grown into SLS, as judged by the electron diffraction patterns. The ZnO thicknesses were fixed at 6 nm per period while MoO$_{\mathrm{3}}$ varied from 2 to 6 nm. The nanostructures as examined by TEM indeed show expected periodicity consistent with XRR. HRTEM also gave clear interfaces of the SLS with certain regions of imperfection. Thence, we conclude that amorphous MoO$_{\mathrm{3}}$ would crystallize when grown adjacent to an initial layer of ZnO. PL was employed to investigate the possible variations of their bandgaps when the constituent ZnO and MoO$_{\mathrm{3\thinspace }}$brought together in close proximity of nanoscakes. Electronic band structures according to ab initio calculations will be discussed.